Thermionic tube amplifier



Jufiy 7, 1942. EL 2,89,

THERMIONIC TUBE AMPLIFIER Filed Jan. 4, 1.941

INVENTOR D. 7'. BELL 5V A T TOR/V5 V Patented July v7, 1942 THERMIONICTUBE ADIPLIFIER- Delamar '1. Bell, Chatham, N. J., assignor to BellTelephone Laboratories, Incorporated, New York, N. Y., a corporation ofNew York Application January 4, 1941, Serial No. 373,086

12 Claims.

This invention relates to multistage thermionic tube amplifiers and moreparticularly to interstage networks 'for coupling together two stages insuch amplifiers.

The principal objects of the invention are to. reduce the cost and sizeof interstagenetworks and to increase the range of transmissioncharacteristics obtainable.

A multistage thermionic tube amplifier requires one or more interstagenetworks to couple the 'output or plate circuit of one tube to the inputor gridcircuit of the next succeeding tube. The chief function ofsuchnetworks is to provide the'required transmission characteristic forthe amplifier. It is also important that the networks be economical asto both cost and space requirements.

'I he interstage network of the present invention isic'apable ofproviding a wide variety of desirable transmission characteristics in acomparatively simple structure which is reasonable in cost and small insize. The network includes two inductively coupled inductors connectedin either the series aiding or the series opposing relationship in ashunt impedancebranchbetween the two tubes to be connected. An impedancebranch, usually including a capacitor and in some cases a resistor, maybe connected in shunt with one or with each of the inductors. Anothercapacitor may be connected in shunt with both 30 of the inductors. .Therequired capacitance may g be partly or wholly furnished by the straycapacitances associated with the component elements and the wiring ofthe amplifier.

The two coupled inductances may be furnished by two separate inductors.To save cost and space, however, a single tapped winding or two windingson the same core may be used. Furthermore, a change in the coefficientof coupling between the inductors, or in the sense ofthe connection,efiects'a' change in the values of the componentimpedance elementsconstituting the network and in practice, these factors are so chosenthat themost advantageous values are secured. This is another importantfactor in reducing the size and cost. In some caseaby the use of coupledinductors, there may be provided transmission characteristics which areotherwiseunderstood from the following detailed descrip-, tion and byreference to the accompanying drawing the single figure of which is aschematic circult of. a multistage thermionic tube amplifier comprisingtwo stages coupled by an interstage network in accordance with theinvention- The amplifier has a pair of input terminals I,

2 upon which are impressed the signals to be amplified and a second pairof terminals 3, 4

to which is connected a load impedance 5 which may include the inputcircuit of a succeeding stage. The amplifier includes a thermionic tube6 having a cathode I, a grid 8 and a plate 9 and a second tube I0 havinga cathode H, a grid l2 l5 and a plate l3. The plate voltage, supplied bythe,battery I5, is impressed on ,thetwo tubes in series. It-isnecessary, of course, that the load 5 have a direct current path for theplate current of the tube In. The amplifier may be grounded, if desired,as indicated at the point 20.

The tubes 6 and III are coupled by an interstage network comprising theshunt impedance branch connected between the point IS, in an \electricalpath between the plate 9 and the grid '25 I2, and the point ll, in'anelectrical path between the two cathodes l and II. The network includesthe series connected inductors L1 and L2 which, in the example shown,are in a path common to the output circuit of the tube 6 and the 5 valueof which may be found from the expression the wiring, and likewise thecapacitance of thecapacitor C: may be partially or entirely suppliedunattainable. This is due to the fact that the 'inherentstrayadmittances associated with the circuit may be successfullyincorporated'into the interstage network, whereas without usinginductive coupling these admittances would be tdo. large for inclusion.

The nature of the invention will be more fully where K is thecoefiicient of the coupling. The inductor L1 is shunted by a capacitorC1 and the .inductor L2 is shunted byan impedance branch Z whichincludes a capacitor C2 and-a resistor R2 connected in parallel. I

The capacitance of the capacitor 01 may be furnished in whole or in partby the stray capacitance associated with the inductor Li, including bythe stray capacitances associated" with the inductor L2, the impedancebranch Z and the. wiring. The capacitor C3 has a comparatively largecapacitance and its function is to block the fiowof direct current whilefreely passing all/ frequencies" within the band of interest. In' an+alyzing the performance of the'interstage network it may, therefore, beconsidered that the points I! and III are common. Thecapacitance of thecapacitor C4 includes the stray capacitance oi the remainder of thenetwork connected between the points l8 and H, the capacitance betweenthe high side wiring and the ground point 20, the capacitance betweenthe cathode I and the plate 9 of the tube 8, the capacitance aaeaoei1100 ohms, which is the proper value to provide the required grid biasfor the tube when the The resistor B may be made variable. as indicatedby the arrow, to allow an adjustment of this voltage. In some cases, ifthe direct current resistances of the inductors L1 and L: are given theproper values, the resistor R may be omitted. It is also obvious thatthe resistor R will be Just as effective for voltage control it placedin series with the inductoraas, for example, it connected between thepoints I! and 2|.

The inductors L1 and La may be connected in either the aiding or theopposing relationship and the coeiiicient of coupling may have any valueup to and including unity. If the series aiding connection is used, as Kis increased in value the number of turns of wire required for theinductors is reduced and as a result the size and cost are decreased andbetter coil constants are secured. Furthermore, the required impedanceof the branches shunting the inductors is decreased. On the other hand,if the series opposing connection is employed, the required battery I5is 150 volts; and therefore the resistance R may be omitted.

What is claimed is:

1. In a multistage amplifier comprising a pluralit of thermionic tubeseach having a cathode, a grid and a plate, an interstage network torcoupling two of said tubes comprising an electrical path between theplate of one of said two tubes and the grid of the other of said twotubes. a second electrical path between the cathodes of said two tubesand a third electrical path connected between a point in saidfirst-mentioned path and a point in said second path, said third pathincluding two inductively coupled inductors connected in the seriesopposing relationship and an impedance branch connected in shunt withone of said inductors.

2. In a multistage amplifier comprising a plurality of thermionic tubeseach having a cathode, a grid and a plate, an interstage network forcoupling two. of said tubes comprising an electrical path between theplate of one of said impedances of the shunt branches are increased.

In practice the sense 01' the connection and the value of K are chosento provide a network which is the most satisfactory as to cost ofcomponent elements and size.

It is to be understood, oi course. that the impedance branches shuntingthe inductors Li and La, represented in the figure respectively by C1and Z, may have any degree of complexity. The

number and type .of the component elements in' each of the branches, andtheirdisposition, depend only upon the transmission characteristic to beprovided by the network. The configurations shown are. to be consideredonly as an illustrative example.

The values of the component elements required for a typical interstagenetwork designed in accordance with the invention will now be presented.The network has the circuit shown in the figure and is capable ofproviding a sub-' The inductors L1 and 1e are connected series two tubesand the grid of the other of said two tubes, a second electrical pathbetween the cathodes of said two tubes and a third electrical pathconnected between a point in said firstmentioned path and a point insaid second path,

ode, a grid and a plate, an interstage network for coupling two of saidtubes comprising an electricalpath between the plate or one of said twotubes and the grid 01' the other of said two tubes, a second electricalpath between the cathodes of said two tubes and a third electrical pathconnected between apoint in said iirstmentioned path and a pointv insaid second path,

aiding and the coeiiicient of coupling K is 0.73.

The value given for C4 is the averagepver the frequency range of 10 to72 kilocycles. The sum 01' said third path including two inductivelycoupled inductors connected in series, an impedance branch including theparallel combination of a capacitance and a resistance connected inshunt with one of said inductors and a second capacitance connected inshunt with both of said inductors. I

' 6. A network in accordance with claim 2 in which said inductors areconnected in the series aiding relationship.

7. A network in accordance with claim 2 in which said inductors areconnected in the series opposing relationship.

8. A'network in accordance with claim 2 which includes a thirdcapacitance connected in shunt with both of said inductors, and in whichsaid inductors are connected in the series opposing relationship.

10. A network in accordance with claim 5 in which said inductors areconnected in series aiding relationship.

11. A network in accordance with claim 5 in which said inductors areconnected in the series opposing relationship.

12. In a multistage amplifier comprising a plurality of thermionictubes, each having a cathode, a grid and a plate, an interstage networkfor coupling two of said tubes comprising two between the cathode andthe grid of other of said tubes, an impedance branch including acapacitance connected in shunt with one of said inductors, a secondcapacitance connected in shunt with the other of said inductors, a thirdcapacitance connected in shunt with both of said inductors and aresistance connected in said inductively coupled inductors connected inseries 10 input circuit in series with said inductors.

in an electrical path which is common both, to

DELAMAR T. BELL.

